A 32-year-old woman presents with fatigue, weight gain, and cold intolerance for 3 months. On examination, she has a firm, diffuse goiter and delayed relaxation of deep tendon reflexes. TSH is elevated at 8.2 mIU/L (normal 0.4–4.0). Which investigation is most appropriate to confirm the diagnosis and determine the etiology?

## Diagnostic Approach to Hypothyroidism with Etiology Determination **Key Point:** When the stem asks to **both confirm the diagnosis AND determine the etiology**, anti-TPO antibodies are the single most appropriate investigation in a young woman with a firm goiter and elevated TSH, as they simultaneously support the diagnosis of primary hypothyroidism and identify Hashimoto's thyroiditis as the cause. ### Why Anti-TPO Antibodies Are the Investigation of Choice Here 1. **Confirms autoimmune etiology:** Anti-TPO antibodies are positive in >90% of Hashimoto's thyroiditis cases (the most common cause of hypothyroidism in iodine-replete regions and in women of reproductive age). A positive result confirms both the diagnosis and the etiology in a single test. 2. **Clinical context strongly points to Hashimoto's:** The triad of firm diffuse goiter + elevated TSH + young woman is the classic presentation of Hashimoto's thyroiditis. Anti-TPO antibodies are the definitive confirmatory test for this condition. 3. **Free T4 alone does NOT determine etiology:** While free T4 confirms overt vs. subclinical hypothyroidism, it does not identify the cause. The stem explicitly asks for the investigation that confirms diagnosis **and** determines etiology — free T4 fulfills only the first part. 4. **Radioiodine uptake** is not indicated in straightforward hypothyroidism and is contraindicated in pregnancy; it is used to evaluate hyperthyroidism or thyroid nodules. 5. **Thyroid ultrasound with FNA** is indicated for nodular disease or suspected malignancy — not for diffuse goiter with elevated TSH. ### Diagnostic Algorithm ``` Elevated TSH + Firm Diffuse Goiter + Young Woman ↓ Anti-TPO Antibodies ↓ Positive → Hashimoto's Thyroiditis (diagnosis + etiology confirmed) Negative → Consider iodine deficiency, drug-induced, or other causes ↓ Free T4 (to grade severity: overt vs. subclinical hypothyroidism) ``` ### Comparison of Options | Investigation | Confirms Diagnosis | Determines Etiology | Appropriate Here? | |---|---|---|---| | Radioiodine uptake scan | No | Partially | No | | Free T4 | Yes (overt vs. subclinical) | No | Partial — does not determine etiology | | **Anti-TPO antibodies** | **Yes (autoimmune)** | **Yes (Hashimoto's)** | **Yes — best single test** | | Ultrasound + FNA | No | Only if nodule present | No | **High-Yield:** Anti-TPO antibodies are positive in ~95% of Hashimoto's thyroiditis and ~75% of Graves' disease. In a young woman with firm goiter and elevated TSH, anti-TPO is the single investigation that answers both "what is wrong?" and "why?" **Clinical Pearl:** Free T4 is the next step to *grade* hypothyroidism severity after the diagnosis and etiology are established with anti-TPO antibodies. The two tests are complementary, but the stem's dual requirement (confirm + etiology) makes anti-TPO the superior single answer. [cite: Harrison's Principles of Internal Medicine, 21e, Ch. 405; Williams Textbook of Endocrinology, 14e]